Experiments will be performed to further characterize and to determine the significance to the tumor-bearing host of a state of concomitant antitumor immunity which is capable of destroying a lethal secondary challenge of tumor cells. The possibility that concomitant immunity, although ineffective against the primary tumor, nevertheless functions to retard the emergence and growth of metastases. This will be studied in T cell-deficient mice which are known not to develop concomitant immunity. Thus, control mice and mice made T cell-deficient by thymectomy and X-irradiation will be implanted subcutaneously with cells of the SA1 sarcoma and the development of concomitant immunity to a standard tumor challenge monitored. It is expected from the results of pilot experiments that failure of T cell-deficient mice to develop concomitant immunity will be associated with a much earlier manifestation of growth of lymph node metastases as monitored macroscopically and by histology. These experiments will be supplemented by those that will determine whether tumor-bearing, T cell-deficient mice display a reduced capacity to destroy SA1 cells injected into the circulation. The loss of 125I from mice given 125I-iododeoxyuridine-labelled tumor cells intravenously will represent one of the methods employed to monitor tumor cell destruction. Similar experiments will be performed with the P-8l5 mastocytoma and the Meth A fibrosarcoma.